The present invention relates to ink jet printing apparatus which includes a printing head, and particularly to a printing head which prevents extreme increases or decreases of ink pressure in the printing head.
A typical example of a known ink jet printing apparatus, known as "drop-on-demand", is illustrated in FIG. 1. In the device illustrated there, a nozzle 2 and an ink chamber 4 are filled with ink delivered from an ink supply 15 through a pipe 14. When an electric pulse is applied to a piezoelectric transducing element 7 from a pulse generator 8, the piezoelectric transducing element 7 moves downwardly and deflects or bends a flexible wall or diaphragm 6 causing a sudden decrease in volume of the ink chamber 4. As a result, liquid pressure in the ink chamber 4 increases suddenly and causes an ink droplet 11 to be ejected through the nozzle 2 whereupon it can be printed as a dot on a printing surface 19.
Ink stored in an ink supply chamber 5 replaces the ink in the ink chamber 4 as a result of a pressure differential between the chambers 4 and 5, pressure in the chamber 4 becoming lower as ink is used up. As the volume of ink in the ink supply chamber 5 decreases, the pressure in chamber 5 also decreases and is detected by a pressure detecting means 9. When the pressure in the ink supply chamber 5 becomes less than a predetermined value P.sub.A, an automatic valve 10 opens and ink 16 in an ink supply source 15 is pressurized by pressurizing means 17 and flows to the ink supply chamber 5 through the pipe 14. When the pressure in the ink supply chamber 5 reaches a predetermined value P.sub.B, the automatic valve 10 closes and terminates flow of ink 16 to the ink supply chamber 5. Thus, the amount of ink in the ink supply chamber 5 is constantly kept at a volume determined by a pressure between P.sub.A and P.sub.B which produces stable ejection of ink droplets 11 from the nozzle 2.
The printing head 1 corresponds to the parts enclosed by the dot-dash lines in FIG. 1 and is connected through a connector 12 to an ink supply system 18 which includes a filter 13, pipe 14, ink supply source 15, and pressurizing means 17. The ejection head 1, after it is manufactured and assembled, usually is filled with liquid (ink) and is stored separately from the printing apparatus with which it is used and is mounted in place thereon when needed. At this time, the printing head 1 is mounted on the printing apparatus and is electrically connected to the pulse generator 8 and to the ink supply system 18 through the connector 12. The inside of the connector 12 is constructed with a self-sealing elastic or rubber device so that no ink leaks out and no air can enter when the connector 12 is attached or detached. As a result of this construction of the printing head 1 and the other component parts associated therewith, replacement of the ejection head is made easy.
There are factors which obstruct normal ejection and flight of ink droplets 11 in these devices. Most frequently, these include air bubbles and solid particles in the nozzle 2 and the ink chamber 4. If air bubbles exist in the nozzle 2 or the ink chamber 4, they can absorb the ejection pressure caused by the actuated diaphragm 6 and can prevent ink droplet ejection, or cause a variance in the flight speed of the droplets, a deviation in the flight path, or may cause the droplets to be split and scattered. Further, if solid particles are in the nozzle 2, normal ejection of ink is obstructed and in the worst case, the nozzle 2 can be clogged. Solid particles in the ejection chamber 4 do not cause immediate problems, but if not prevented usually result in eventual clogging of the nozzle.
Bubbles and solid particles in the ink chamber and nozzle occur, inter alia, when the printing head 1 is subject to unusual impact forces which can cause bubbles to be formed in the ink chamber 4 and/or in the nozzle 2, or when the ambient temperature varies to an extent which causes expansion and contraction of ink in the printing head 1 and resultant bubble formation. Solid particles can be formed by ink in the nozzle 2 which dries and sets when the printing head 1 is unused for a long time or when the ambient temperature is unusually low.
To avoid these problems, it has been proposed that the printing head be filled with a liquid during storage. Such filling liquids include ink from which dye and pigment have been removed, and liquids which are chemically stable and resist evaporation. This measure reduces the likelihood of solid particles in the nozzle 2 and the likelihood that air will be drawn into the nozzle 2 as a result of evaporation of the filling liquid and the formation of air bubbles. In addition, soft rubber has been applied to the outlet of the nozzle 2 with pressure and the automatic valve 10 is maintained closed so that the printing head 1 is perfectly sealed.
The foregoing measures make it possible to prevent inhalation of air through the nozzle and resultant air bubble formation by nominal thermal expansion and contraction of the filling liquid and by unusual impacts on the printing head, as well as the formation of solid particles and bubble formation by evaporation of filling liquid. However, the ambient temperature can reach a level at which the filling liquid expands or contracts to an extent that causes a pressure change in the printing head which exceeds the capability of the seals. At exceedingly low temperatures, liquid pressure in the printing head can drop to a point where the decrease in liquid volume causes damage of structural parts of the printing head. For example, adhesives used to join some of the printing head parts can weaken and be damaged when the filling liquid pressure drops to a very low level. At very high ambient temperatures, the filling liquid expands and can cause an internal pressure rise in the head which also can cause structural part damage.
Furthermore, when a constitutional element with a high transmission factor for the filling liquid is used in the printing head, the filling liquid in the printing head evaporates with time while stored. Again, the internal pressure in the printing head can drop to a point where damage can occur.
The problems described above are not limited to storage conditions but also can arise when the printing head is mounted on the printing apparatus. Thus, when the power is off and the device is not being used, the influence of external temperature changes, evaporation of the filling liquid, etc. are nearly the same as when the printing head is in storage. This is so because it is important at this time that the nozzle 2 be sealed and the automatic valve 10 be closed. Furthermore, at this time, the printing head is filled with ink which, if allowed to dry and set, forms undesirable solid particles.
FIG. 2, which is an enlarged sectional view of FIG. 1 taken along the line A--A, shows a conventional device in which the ink supply chamber 5 has an upper wall formed by a flexible film 21 and having an elastic plate 20 disposed therein and connected to a strain gauge 9. When the liquid pressure in the supply chamber 5 changes as ink is used up, the elastic plate 20 deforms slightly. The deformation is sensed by the strain gauge 9 which in turn controls (opens or closes) the valve 10. However, when the volume of ink in the supply chamber changes abnormally, for example, as a result of a large temperature change, evaporation of ink through the flexible film 21, etc., the elastic plate 20 can deform to a point where the strain gauge 9 is damaged.
Therefore, a main object of the present invention is to provide an ink printing head wherein problems caused by change in the volume of ink or other liquid in the printing head are prevented.